Natural selection and variation - What are the conditions for natural selection?

The conditions for natural selection

The excess fecundity, and consequent competition to survive in every species, provide the preconditions for the process Charles Darwin called natural selection. Natural selection is easiest to understand, in the abstract, as a logical argument, leading from premises to conclusion. The argument, in its most general form, requires four conditions:

1. Reproduction. Entities must reproduce to form a new generation.

2. Heredity. The offspring must tend to resemble their parents: roughly speaking, 'like must produce like'.

3. Variation in individual characters among the members of the population. If we are studying natural selection on body size, then different individuals in the population must have different body sizes.

4. Variation in the fitness of organisms according to the state they have for a heritable character. In evolutionary theory, fitness means the average number of offspring left by an individual relative to the number of offspring left by an average member of the population. This condition therefore means that individuals in the population with some characters must be more likely to reproduce (i.e., have higher fitness) than others.

If these conditions are met for any property of a species, natural selection automatically results. And if any are not, it does not.

Thus entities, like planets, that do not reproduce, cannot evolve by natural selection; entities that reproduce but in which parental characters are not inherited by their offspring also cannot evolve by natural selection. But when the four conditions apply, the entities with the property conferring higher fitness will leave more offspring, and the frequency of that type of entity will increase in the population.

Richard Dawkins gives his own definition of the conditions for natural selection.